Among semi-active suspension systems installed in vehicles, for example, there is known a semi-active suspension system that employs a damping force control type hydraulic shock absorber having a damping valve mechanism incorporated in a piston (for example, see Patent Literature 1). Conventionally, such a damping force control type shock absorber is arranged such that a cylinder upper chamber and a cylinder lower chamber are always in communication with each other through a passage for expelling unwanted air from a hydraulic circuit. Therefore, the conventional damping force control type shock absorber has drawbacks such as delay in rising of damping force, and limitation on the damping force adjustable range.
Under the above-described circumstances, it may be conceivable to reduce the sectional area (flow path area) of the passage for expelling air to thereby minimize the delay in rising of damping force and the reduction in damping force adjustable range. With this scheme, however, the manufacturing cost increases because tolerances become tighter. To solve this, it may be conceivable to provide a check valve in the above-described passage to thereby block the flow of hydraulic fluid from the cylinder upper chamber toward the cylinder lower chamber, as shown, for example, in Patent Literature 1. This, however, results in a complicated structure and leads to an increase in the number of assembling man-hours and an increase in manufacturing cost.